what component fails in the pcb ofter just 4 years, bearing in mind Im an off-shore electrician and worked away 2 years of that period???
If its not posible to repair, whats the cost to replace?
well assuming you've already established it is the pcb....then replace it, there's bugger all else you can do with them any wolsley parts center will give you the price.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix home heating systems.